Envelope tracking systems for wide bandwidth modulation systems may be sensitive to the time-alignment between the modulated power amplifier supply voltage provided to a radio frequency power amplifier and a modulated radio frequency envelope signal provided as a signal input of the radio frequency power amplifier. A delay misalignment between the modulated power amplifier supply voltage and the radio frequency modulated envelope signal provided may result in spectrum and error vector magnitude degradation at an output of the radio frequency power amplifier. As an example, in the case of an LTE signal having a 10 MHz channel bandwidth, the spectrum requirements cannot be met when the absolute delay mismatch between the modulated power amplifier supply voltage and the radio frequency modulated envelope signal received by the radio frequency power amplifier is greater than four nanoseconds.
In addition, because detectors used to measure an output power at an output of the radio frequency power amplifier include offset errors and suffer from limited or low sensitivity with respect to delay mismatch, calibration techniques based on minimizing mismatch delay by determining an offset delay that provides maximum output power may not be repeatable or reliable.
Accordingly, there is a need for a new system and technique for configuring and calibrating open loop envelope tracking systems to minimize group delay misalignment between the modulated power amplifier supply voltage and the radio frequency modulated envelope signal. In addition, there is a need to develop a new system and technique for configuring an open loop envelope tracking system during factor calibration or phone operation that minimizes or eliminates special external factory equipment or complex internal circuitries inside a communication device.
Furthermore, in an envelope tracking system, where a digital predistorted envelope signal is generated to modulate a PA supply, the input sample rate of the digital signal vramp is variable, based upon the modulation format and bandwidth. However, the final sample rate is always the same. This requires many interpolation rates to be implemented. Very fine timing control of the vramp delay is also required (as fine as 0.5 nanoseconds), where vramp is a digital signal representing the predistorted envelope signal. The fine delay control and many interpolation ratios can lead to the need or several very large coefficient look up tables (LUTs) to perform all of the required rate conversions and fractional delays. These large LUTs require a lot of digital area, consume large amounts of power, and may not be realizable from a digital timing perspective. Accordingly, there is also a need for an improved method and apparatus to decrease the size and power consumption of a rate converter in an envelope tracking system.